The events first triggered by fertilization involve ionic changes. There is an immediate change in the plasma membrane's ion permeability, as well as a dramatic change in intracellular ion concentrations. These ionic events must be important steps in the activation of development because simple manipulations of the egg's ionc environment will often activate development parthenogenetically. This research will study the role of both transcellular ion currents and intracellular ion concentration changes in activation and ooplasmic segregation in two vertebrate eggs, that of the frog, Xenopus laevis, and the fish Oryzias latipes, using a number of new techniques: Avibrating probe technique will be used to electrically detect the extracellular ion currents triggered by fertilization. Ca++, H+, Na+, and Cl-ion concentrations will be studied using intracellular ion-specific microelectrodes. One of the first developmental events occurring after fertilization is a dynamic cytoplasmic redistribution termed "ooplasmic segregation." Since there is evidence that this process is also influenced by ion currents and gradients, a study of the roles of these early ionic changes in segregation would be a natural extension of this research. The steady transcellular ion currents which the plasma membrane drives through the egg during this segregation will be mapped and the ionic components will be identified. A major goal of this work is to determine if these currents play a casual role in the segregation mechanism.